Accurate determination of the temperature of the gas sample blown by the subject into the measuring device is necessary for the measurement of the breath alcohol concentration. However, the temperature sensor cannot be introduced directly into the area of the mouth of the subject for hygienic and application technical reasons. It is located, instead, behind a replaceable mouthpiece, which is inserted into a heated holder on the measuring device. To avoid condensation effects of the gas sample, which is saturated with moisture, within the holder, the holder and usually also the components arranged downstream are heated to a temperature of about 39.degree. C.
A mouthpiece, into which the subject blows, is inserted into the holder before the beginning of the measurement. Since a plurality of measurements are normally performed with different subjects in a rapid succession, the mouthpiece is inserted into the holder only shortly before the measurement. The temperature of the mouthpiece is normally at the ambient temperature level before use, and this temperature level is subject to great variations depending on the place of use and the time of the year. If the mouthpiece has not completely assumed the temperature of the holder before the measurement, the temperature measurement is distorted, because the breath gas sample is cooled by the mouthpiece during the blowing into the breath alcohol-measuring device. Since it is, on the other hand, impossible to wait until a stable temperature gradient has become established between the mouthpiece and the heated holder, knowledge of the temperature of the mouthpiece at the time of insertion into the holder is necessary.
A sampling device of a breath alcohol-measuring device, in which a mouthpiece is inserted into a preheated holder, has been known from the publication "Schonknecht, G. and B. Stock: The Technical Concept for Evidential Breath Testing in Germany", 13th International Conference on Alcohol, Drugs and Traffic Safety, Adelaide, Aug. 13 through Aug. 18, 1995. However, the temperature sensor is located in a sampling channel, which is surrounded by the wall of the holder. When the mouthpiece is pushed into the holder, the temperature at the temperature sensor changes but slightly, because the temperature sensor is shielded from the mouthpiece by the wall of the sampling channel. The effect of the storage temperature of the mouthpiece on the temperature measurement of the gas sample flow has hitherto been underestimated, because the mouthpiece has a very small mass and, on the other hand, it also has a length of only a few cm. However, more recent investigations have shown that the accuracy of measurement can be markedly increased by taking into account the temperature of the mouthpiece during the breath alcohol analysis.